1. Field of the Invention
Embodiments of the invention relate to the field of optical communication devices. More particularly, the present invention relates to a transmitter used for optical communication systems implemented on a photonic integrated circuit.
2. Discussion of Related Art
Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs. Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have been packaged separately and mounted on a printed circuit board. More recently, however, many WDM components including transmitter, receivers and passive devices have been integrated onto a single chip also referred to as a photonic integrated circuit (PIC). In order to further increase data transmission rates associated with WDM systems, various modulation formats have been proposed for generating the modulated output. In accordance with one such format, different information is modulated on to optical signals having the same wavelength, but different polarizations. The optical signals are then polarization-multiplexed (pol-muxed) and combined with other pol-muxed optical signals onto a common optical fiber. One approach for combining such pol-muxed optical signals involved combining optical signals having a first polarization with a first WDM combiner to provide a first WDM signal and combing optical signals having a second polarization with a second WDM combiner to provide a second WDM signal. The first and second WDM signals are then combined with a polarization beam combiner (PBC). Integration of multiple WDM combiners on a PIC, however, complicates the design of the PIC and may reduce yields. Accordingly, a simpler design having fewer WDM combiners is advantageous.